FIG. 1 diagrammatically illustrates a conventional telecommunication transceiver of the type that may be employed for processing HDSL2 signals, including performing echo cancellation, received from a telecommunication wireline pair. The transceiver incorporates Tomlinson preceding and a T/2-spaced receiver equalizer. Because such preceding is not compatible with the traditional decision feedback (DFB) methods used with original HDSL technology, the conventionally employed receiver DFB algorithm has been replaced with an alternative nonlinear technology—a Tomlinson precoder, in the transmitter. See, for example, the articles entitled: “Matched-Transmission Technique for Channels With Intersymbol Interference”, by H. Harashima et al, IEEE Transactions on Communications, Vol. COM-20, No. 4, August, 1972, and “New Automatic Equaliser Employing Modulo Arithmetic,” by M. Tomlinson, Electronics Letters, Vol. 7, No. 5, March 1971.
In the transceiver of FIG. 1, the incoming signal from an upstream analog front end (not shown) is digitized by an analog-to-digital converter 10 and sampled by way of a fractionally spaced (e.g., two samples per symbol) sampling operator, represented by a T/2 switch 20. The output of the T/2 sampling switch is coupled to a first (+) input 31 of a subtraction operator 30. A second (−) input 32 of the subtraction operator is coupled to the output of an echo canceler 40 by way of (T/2) fractionally spaced sampling operator 50. Echo canceler 40 is coupled to receive the transmitted (TX) signal that is originally applied to the wireline pair. (As pointed out above, the transmitted signal is shown as being subjected to a pre-encoding (e.g., Tomlinson encoding) operator 60.)
The output 33 of the subtraction operator 30 is sampled by way of a further fractionally spaced (T/2) operator 70, the output of which is coupled to a linear equalizer 80, and also fed back as an error signal E1 that is used to update the coefficients of the echo canceler 40. The output of the equalizer is coupled through a unit (one sample per symbol) operator 100 for application to a decision device (e.g., slicer) 110. Due to the presence of the Tomlinson precoder 60 in the signal path of the far-end transmitter (not shown), the output of the linear equalizer 80 is subjected to a modulo-decoder 120 prior to being coupled to the decision slicer 110. The taps of the equalizer 80 are updated by subtracting the output of the slicer 110 from its input in a subtraction operator 130 to produce an error signal E2. This error signal is coupled to the linear equalizer 80 by way of a T-spaced sampling operator 140.
In the transceiver of FIG. 1, the Tomlinson precoder provides an equivalent function of a decision feedback (DFB) equalizer. Due to the incompatibility of the DFB and the preceding used in HDSL2, it is necessary to use this nonlinear preceding at the far-end transmitter to replace the DFB at the local receiver. This results in the use of a nonlinear modulo device 120 after the linear equalization, in order to undo the nonlinear preceding at the far-end transmitter. A start-up coefficient exchange protocol is also needed to convey the DFB information from the local receiver to the far-end transmitter.
These developments have mandated changes in some other receiver algorithms. Among these are methods of providing echo canceler updates during initial training and during data mode when the nonlinear precoder is enabled.
In the receiver and echo cancellation architecture of FIG. 1, the error signal E1 for updating the coefficients of the echo canceler is taken immediately after the point of echo cancellation. This error signal offers a couple of advantages over other choices. First, it is available for each sample at which echo cancellation occurs—in this case, at a T/2 rate. Second, it is available immediately, so that there is no delay in processing the updates. Unfortunately, it has one major disadvantage: the error signal taken immediately after the point of echo cancellation contains the desired received far-end signal, in addition to the residual echo error and loop noise.
Under proper operating conditions, the echo signal must be suppressed far below the level of the received far-end signal, preferably substantially below the level of received background noise, in order to avoid losing margin performance due to residual echo. As a consequence, after echo canceler convergence when the system is operating properly, the error signal that functions to update the echo canceler should be buried to a depth well over 30 dB below the desired received far-end signal. This kind of cancellation is necessary in order to ensure that the echo canceler does not compromise margin performance; however, the received far-end signal appears as noise or interference to the echo canceler update algorithm.
Because the echo canceler update error signal appears noisy (being covered up by the received far-end signal) after the error converges to the low residual echo levels required, the update gain for the echo canceler must be set extremely low, in order to filter out the noise and provide accurate updates. Unfortunately, at very low gain levels the echo canceler taps are only able to adapt at a very slow rate. Thus, the echo canceler cannot respond rapidly to changes in the echo. If such changes occur, they lead to reduced echo cancellation and diminished performance (loss of margin or errors or even receiver failure) until the echo canceler can re-converge.
Some recent developments in wireline communication technology have highlighted problems associated with the slower echo canceler update. It appears that rapid changes in the loop may result in sudden changes in the echo that require more rapid echo canceler adaptation. The solution requires an error signal that can update two samples per symbol, but at the same time be sufficiently noise-free to allow much more rapid updates.